In general, a strut-type suspension is mainly used in a front wheel of a four-wheeled motor vehicle, and is constructed such that a strut assembly incorporating a hydraulic shock absorber in an outer cylinder formed integrally with a main shaft is combined with a coil spring. Among such suspensions, there is a type of structure in which the axis of the coil spring is actively offset with respect to the axis of the strut, so as to allow the sliding of a piston rod of the shock absorber incorporated in the strut to be effected smoothly. There is another type of structure in which the coil spring is disposed by aligning the axis of the coil spring with the axis of the strut. In either structure, a bearing is disposed between a mounting member for a motor vehicle body and an upper spring seat of the coil spring to allow the rotation to be effected smoothly when the strut assembly rotates together with the coil spring by the steering operation.
In this thrust bearing, a rolling bearing using balls or needles or a synthetic resin-made sliding bearing is used. However, the rolling bearing has a possibility of causing a fatigue failure in the balls or needles owing to such as infinitesimal oscillations and a vibratory load, so that there is a problem in that it is difficult to maintain a smooth steering operation. As compared with the rolling bearing, the thrust sliding bearing has a high frictional torque and therefore has the problem that the steering operation is made heavy. Furthermore, both bearings have the problem that the steering operation is made heavy due to the high frictional force of a dust seal formed of a rubber elastomer fitted to prevent the ingress of foreign objects such as dust onto sliding surfaces, and the synthetic resin-made sliding bearing in particular has the problem that the steering operation is made much heavier.
To overcome the above-described problems, the present applicant proposed synthetic resin-made thrust sliding bearings which are each comprised of a synthetic resin-made upper casing, a synthetic resin-made lower casing, and a synthetic resin-made bearing piece interposed between the upper and lower casings, wherein the upper and lower casings are combined by elastic fitting, and a resiliently fitting portion and a sealing portion based on labyrinth action are respectively formed between the upper and lower casings and between an inner peripheral surface side and an outer peripheral surface side, to prevent the entry of foreign objects such as dust onto the bearing sliding surface by means of that sealing portion (described in Patent Document 1, Patent Document 2, Patent Document 3, Patent Document 4, and Patent Document 5).
This thrust bearing will be described with reference to drawings as follows. In FIGS. 12 and 13, a synthetic resin-made thrust sliding bearing 1 is disposed between a vehicle body-side mounting member X fixed to the vehicle body side by means of a mount insulator and a flat upper surface Q1 of an upper spring seat Q disposed in such a manner as to oppose a lower surface X1 of the vehicle body-side mounting member X and surround an outer peripheral surface P1 of a piston rod P fixed at its one end to the vehicle body-side mounting member X. This synthetic resin-made thrust sliding bearing 1 is comprised of a synthetic resin-made upper casing 100, a synthetic resin-made lower casing 200, and a synthetic resin-made bearing piece 30 interposed between the upper and lower casings 10 and 20. The upper casing 10 includes: an annular plate portion 12 having a circular hole 11 in its central portion, a cylindrical suspended portion 14 formed integrally with a lower surface 13 of the annular plate portion 12 in such a manner as to be radially outwardly spaced apart a predetermined interval from a peripheral edge of the circular hole 11; a cylindrical engaging suspended portion 16 formed integrally at an outer peripheral edge of the annular plate portion 12 in such a manner as to be radially outwardly spaced apart a predetermined interval from the cylindrical suspended portion 14, so as to form an annular groove 15 in cooperation with the cylindrical suspended portion 14; and an engaging hook portion 17 formed on an inner peripheral surface of an end portion of the cylindrical engaging suspended portion 16. The lower casing 20 includes: an annular plate portion 22 having in a central portion thereof an insertion hole 21 communicating with the circular hole 11 of the upper casing 10; an annular projection 23 having an inside diameter identical to that of the insertion hole 21 and formed integrally on an upper surface of the annular plate portion 22; an annular projection 25 formed integrally on the upper surface of the annular plate portion 22 in such a manner as to be radially outwardly spaced apart a predetermined interval from the annular projection 23, so as to form an annular recess 24 in cooperation with the annular projection 23 and the upper surface of the annular plate portion 22; a cylindrical engaging projecting portion 27 formed integrally at an outer peripheral edge of the annular plate portion 22 in such a manner as to be radially outwardly spaced apart a predetermined interval from the annular projection 25, so as to form an annular groove 26 in cooperation with the annular projection 25; an engaged portion 28 formed on an outer peripheral surface of the cylindrical engaging projecting portion 27; and a cylindrical portion 29 having an inside diameter identical to that of the insertion hole 21 and formed integrally on the lower surface of the lower casing 20. The upper casing 10 is combined with the lower casing 20 by causing the engaging hook portion 17 to be resiliently fitted to the engaged portion 28 of the lower casing 20.